This invention relates generally to an operating level compensation circuit for compensating an operating level of a high gain amplifier to amplify a very small signal, for instance, a small electrical signal converted from a very small optical signal, and more particularly to a circuit for compensating an operating level of an amplifier including an alternative current amplifier (AC amplifier) which amplifies such a small signal, to keep always a zero level of the signal being constant in spite of various signal waveforms and amplitudes in order to improve a linearity of the amplifier and to suppress fluctuations of a dynamic range of the amplifier.
Recently, top engineerings such as optical fiber communication and so forth have been remarkably developed by supporting of high speed semiconductor technique. In such engineering fields, a wide bandwidth, low noise and high gain amplifier has been more required and it has been important problem to develop the high performance amplifier.
The optical signal has been converted to the electrical signal to be amplified, wherein the high gain amplifier has been required, because the signal has generally been very small. It has been better in not only a performance but a cost to employ the AC amplifier having an enough low cut off frequency than to use cascaded direct current amplifiers (DC amplifiers) including an unavoidable inherent problem of DC drift.
In the optical fiber communication and so forth, the AC amplifier, which has had no problem of the drift and is operatable with a single power supply, miniaturizable and minimizable in power consumption, has been used, because it has not been needed to employ DC amplifiers.
However, in an optical signal measurement and so forth, for instance, in an optical time domain reflectometer (OTDR), it has sent optical pulses from a terminal of a fiber, has amplified and displayed reflective waves from many points of the fiber in order to measure characteristics and to locate disconnections of the fiber, wherein it has been needed to accurately detect a zero level of faint signals of the reflective waves and to use DC amplifiers.
Although, the DC amplifiers including the drift problem and a difficulty of cascaded connections, and requiring plural power supply, have not been always suitable for amplifying the faint signals.
Particularly, in the OTDR, it has been an important subject to get a wide dynamic range amplifier for amplifying the both extremes of the very big amplitude signal from vicinity of a measuring terminal and the faint signal from a distant portion.
In the OTDR, a logarithmic amplifier for amplifying back scattering waves caused by the Rayleigh scattering of the fiber and the reflective waves caused by the Fresnel reflection from discontinuities such as junction points of fibers, a disconnecting point and so forth, has been used in order to get a wide dynamic range and to measure attenuation characteristics of the fiber.
In the logarithmic amplifiers, for reasons to avoid the drift and to get high gain, the AC amplifier has been conventionally employed.
Thereupon, in the OTDR, for the reasons above, the wide band, high gain and wide dynamic range amplifier has been required. However there are disadvantages that such a conventional AC amplifier has been not only expensive but disadvantageous in its characteristics that its dynamic range and accuracy have been seriously influenced by fluctuations of its operation level due to variations of input signal waveforms, for instance, variations of a duty cycle of input pulses.
Especially, in the high gain logarithmic amplifier comprising cascaded logarithmic amplifiers coupled by capacitors between them, the fluctuations of the operation level have caused remarkable deterioration of measurement accuracy due to the logarithmic characteristics, therefor good performances have not been obtainable unless compensations to stabilize the operation level.